Birds lay eggs. They have feathers which covered the body and two forelimbs are modified for flight. But mammals have mammary glands for the production of milk to nourish their young. The skin has hairs as well as sweat and oil glands. Most mammals familiar to us produce live young ones. However, a few of them, like the platypus and the echidna lay eggs. Thus there is difference between Aves and mamalia.
